Off-grid systems get sized backwards constantly. People start from panel wattage because that is the number on the box, when the figure that actually determines everything is how many watt-hours you consume in a day.
This calculator works in the correct order. Enter what you run and for how long, and it works out the daily total first, then sizes the inverter, battery bank, and array from that. Our note on how much solar a cabin needs covers the same math in longer form.
What Each Output Means
| Output | How it is calculated |
|---|---|
| Daily use | Watts times hours, summed across every appliance |
| Inverter | Total connected watts plus 25 percent headroom |
| Battery bank | Daily watt-hours times autonomy days, divided by usable depth |
| Solar array | Daily watt-hours divided by peak sun hours, plus 30 percent for losses |
Start With Watt-Hours
Watts describe how fast something draws power. Watt-hours describe how much it consumes over time, and time is what a battery has to cover overnight. The calculator needs a daily total first, which building an off grid load list produces line by line.
A 50 watt device running four hours uses 200 watt-hours. A 500 watt device running twelve minutes uses 100 watt-hours. The second draws ten times harder and consumes half as much.
That is why a list of appliance wattages tells you almost nothing on its own, and why the calculator asks for hours alongside watts.
Being honest about hours matters more than being precise about watts, because people consistently underestimate how long things actually run.
Refrigeration is the usual surprise. A compressor cycles day and night, so it accumulates far more watt-hours than its wattage suggests. Our roundup of 12V refrigerators covers the efficient options.
Why the Inverter Number Has Headroom
It sizes to simultaneous load
The total watts of everything that could run at once.
The 25 percent margin is standard
Running an inverter at its ceiling continuously shortens its life.
Surge is a separate rating
Motors draw several times running watts at startup.
Check surge against your largest motor
Pumps and compressors are the ones that matter. Our roundup of power inverters covers both ratings.
Battery Chemistry Changes the Answer Substantially
The calculator asks for chemistry because usable capacity differs enormously between the two common types.
Lithium iron phosphate delivers roughly 80 percent of its rated capacity before you are into territory that shortens its life.
Lead acid delivers roughly 50 percent, which means a lead acid bank has to be substantially larger to provide the same usable energy.
Switching the selector from LiFePO4 to lead acid on the same load shows that difference immediately, and it is the reason lithium dominates off-grid storage despite the higher purchase price.
System voltage matters too. The same energy at 48V requires a quarter of the amp-hours it would at 12V, which is why larger systems run at higher voltages. Our note on LiFePO4 versus lead acid covers the comparison.
Days of Autonomy Is the Expensive Setting
Autonomy is how many days the bank carries the load with no meaningful generation, and it multiplies the battery requirement directly.
One day means a single cloudy stretch drains you. Three days is the common recommendation where running out is genuinely disruptive.
Each additional day adds the full daily consumption to the bank, which is where the cost sits in most off-grid builds.
That is also the argument for oversizing panels instead. Panels are cheaper per unit of margin, and an oversized array simply throttles when the bank is full.
Two days is a reasonable default for a system with a generator as backup, and three suits a setup with no fallback. Our note on how many batteries you need covers the arithmetic.
Peak Sun Hours Is Not Daylight Hours
Peak sun hours is the equivalent number of full-output hours a location receives, not the number of hours the sun is up.
A location with fourteen hours of daylight in June might have five peak sun hours, because output ramps up and down rather than running flat.
The figure varies enormously by season and latitude, and winter values in northern locations can be under half the summer number.
Sizing on an annual average produces a system that works for part of the year and fails in the months you most need it.
The honest approach is entering the figure for the worst month you intend to use the system, which is what the calculator assumes you are doing. Our note on choosing solar panels covers output ratings.
What the 30 Percent Array Margin Covers
The array figure includes a loss margin because real systems never deliver their theoretical output.
Charge controllers lose a percentage in conversion, and MPPT controllers lose less than PWM ones.
Wiring loses energy as heat, and that loss scales with run length and inversely with wire gauge.
Battery charging is not 100 percent efficient either, so some of what arrives at the bank never comes back out.
Panel output also drops as cells heat up, which means a hot summer day produces less than the rated figure suggests. Our note on sizing a charge controller covers the conversion stage.
Where the Calculator Stops
This sizes the four main components and it does not design a system.
Wire gauge depends on current and run length, and undersized cable is a fire risk rather than an efficiency problem.
Fusing has to protect the wire rather than the device, and battery banks need protection close to the terminal.
Panel configuration matters too. Series raises voltage and parallel raises current, and the controller has limits on both.
Treat the outputs as a starting specification to check against real products rather than a finished design. Our note on what size fuse you need covers protection.
Reading the Result Sensibly
The outputs are a specification rather than a shopping list, and the gap between the two is worth understanding.
Batteries come in standard capacities, so a bank calculated at 320Ah means buying whatever combination of real units reaches or exceeds that.
Inverters come in standard sizes too, and rounding up to the next available rating is correct rather than wasteful.
Panels are sold in fixed wattages, and an array target of 500W might be two 250W panels or four 125W ones depending on what suits your mounting and your controller.
Where a figure sits just above a standard size, it is worth checking whether trimming a load brings it under, since that can save a meaningful amount of money. Our roundup of 100Ah LiFePO4 batteries covers common bank building blocks.
Common Sizing Mistakes
Starting from panel wattage
Panels are the output of the calculation rather than the input.
Using rated battery capacity
Lead acid delivers roughly half its rating in practice.
Entering summer sun hours for a year-round system
Winter values can be under half, and that is when you need it most.
Underestimating running hours
Refrigeration runs constantly, which is why it dominates most off-grid loads.
Recommended Reading
See our note on running a cabin entirely on solar, our note on what appliances you can run off grid, our roundup of charge controllers, and a note on choosing a solar battery.
Solar Sizing Calculator FAQ
What does this calculator do?
Takes your appliance list and works out daily watt-hours, then sizes an inverter, battery bank, and solar array from that figure.
Why does it ask for hours as well as watts?
Because watt-hours rather than watts determine what the battery has to cover. A high-draw device running briefly consumes less than a modest one running all day.
Why does battery chemistry change the result so much?
Usable depth differs. LiFePO4 delivers roughly 80 percent of rated capacity, lead acid roughly 50 percent, so a lead acid bank has to be substantially larger.
What are peak sun hours?
The equivalent number of full-output hours, not daylight hours. A location with fourteen hours of daylight might have five peak sun hours.
How many days of autonomy should I choose?
Two is reasonable with a generator as backup. Three suits a system with no fallback. Each day added multiplies the battery requirement.
Why is there a 30 percent margin on the array?
Controller conversion, wiring resistance, and charging inefficiency all take a share, and panels produce less as they heat up.
Does this account for surge?
Only partly. The inverter figure includes 25 percent continuous headroom. Motors draw several times running watts at startup, so check surge rating against your largest motor separately.
Can I use this to design a full system?
Treat it as a starting specification. Wire gauge, fusing, and panel string configuration all need working out separately before anything gets built.
